14A. Motor neuron diseases

Page created on June 3, 2021. Not updated since.


Motor neuron diseases are diseases which selectively affect motor neurons in the spinal cord and motor cortex. They’re progressive diseases which lead to severe disability and death.

The most important types are:

  • Amyotrophic lateral sclerosis (ALS)
  • Hereditary spastic paraplegia
  • Spinal muscular atrophy (SMA)

ALS occurs sporadically, while spinal muscular atrophy and hereditary spastic paraplegia are genetic disorders.

Clinical features

In ALS both upper and motor neuron lesions are present simultaneously. The patient initially has weakness in the hands, feet, or bulbar muscles (causing dysarthria or dysphagia), but later progresses to having symptoms in all muscles. They can develop “drop head” due to weak neck muscles.

In hereditary spastic paraplegia only upper motor neuron lesion is present. Only the lower limbs are affected, causing spastic gait and weakness.

In spinal muscular atrophy only peripheral motor neuron lesion is present. The symptoms depend on the age of onset, which can be anything from early infancy to young adults. All muscles can be affected.

Diagnosis and evaluation

Physical examination to detect lower and upper motor neuron lesions is important. LMN lesion can be seen as muscle atrophy and fasciculations, while UMN lesion can be seen as hyperreflexia, positive Babinski, and spasticity.

Lower motor neuron lesions can be identified by EMG, while upper motor neuron lesions can be identified by MRI or motor evoked potentials (MEP).

Genetic testing confirms hereditary spastic paraplegia and SMA.

It’s important to exclude other neurological disorders.


The treatment is mostly supportive. The patient eventually becomes unable to swallow and breathe, which require enteral feeding and ventilation.


The prognosis of ALS is very poor. Most survive only a few years. The prognosis of hereditary spastic paraplaegia is good, and the prognosis of SMA depends on the age of onset (the earlier onset the worse prognosis).

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13B. Clinical features of multiple sclerosis

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14B. Diagnosis of multiple sclerosis

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